Ultrasound is commonly used to measure distances to an object. In many embodiments this measurement is achieved by the transmission of an ultrasonic pulse from a transmitter and subsequently timing how long it takes for the reflection of that pulse to be received from the object. Often a transceiver is used wherein the transmitter is used both to generate the ultrasonic pulse and receive the reflection. The time that the pulse has taken to reach the object and return can be used to calculate the distance to the object if the speed of sound in the medium in which the sound pulse has been transmitted is known.
The accuracy of this technique is affected by various factors which include the component fluids, which would generally be gases, which make up the medium in which the sound pulse travels and the temperature of the medium. If either the component fluids or the temperature varies then the speed of sound changes and the calculation is inaccurate.
It is often necessary to measure the level of a liquid within a container and such level measurement provides one example of a distance measurement which has been made using ultrasound. In some situations, such as in a tank, this is useful to determine the volume of liquid in the container. In other situations, such as within a primary element of a flume or weir, this is useful to determine the rate at which fluid flows through the primary element.
In addition to measuring the level of fluid in a container or flume it is also known to use ultrasound to measure the distance to an object and be incorporated into devices such as collision detectors, distance measuring devices and the like.
In one particular application, given by way of example only but which is convenient to describe, measuring the rate of flow is becoming more important as companies are being monitored more closely as to how much waste they discharge and are being charged accordingly. It is therefore desirable to be as accurate as possible.
As discussed above one prior art technique of determining a level is to use an ultrasonic pulse. However, in an outside environment the temperature, and thus the speed of sound in air, varies by on the order of 50° Celsius over the operating conditions which may be experienced.
Techniques have been proposed which try to reduce this problem. Such techniques have included providing temperature sensors. However, simply providing a temperature sensor does not provide a suitable solution since direct radiated heat from the sun may cause a false reading, there may be too much lag in the reading taken by the sensor, etc.
It is known to provide shielding to shield the sensor from direct sun light however, such shielding can itself provide a further thermal mass which increases the lag experienced by the sensor.
It is also known to provide reference distances, by provision of a peg or the like. The reference distance can allow any measurements that have been taken to be corrected for temperature variation; since the reference distance is known then readings can be adjusted until calculations give the correct distance for the reference distance. However, such techniques are only accurate whilst the reference distance is accurate. Foreign bodies can become deposited on the peg, etc., for example, when a flood occurs, ice may form on the peg, etc. all of which can seriously affect the accuracy of readings taken.